Nuclear receptors for progesterone and estradiol in the guinea pig uterine compartment during gestation.

Because progesterone suppresses myometrial contractility, the assumption is often made that the withdrawal of this steroid is a prerequisite for parturition. However, steroid patterns in maternal blood of the guinea pig do not consistently change with impending parturition and it has been claimed that progesterone does not suppress guinea pig myometrial contraction. The present study investigated progesterone and estrogen nuclear receptor binding in myometrium, endometrium, and chorion between 32 days of gestation and delivery at 67-71 days. Binding characteristics and behavior during sedimentation in sucrose density gradients were typical of the steroid hormone receptor family. Decreased progestin binding occurred in the myometrium, from a high of 1600 fmol/mg DNA at 49-51 days to a low of 450 fmol/mg DNA (P < 0.01) on the day of detectable pubic relaxation. This decrease commenced at 60-63 days just before the onset of relaxation. A similar, though less well defined change occurred in endometrium. Estradiol nuclear receptor binding in myometrium remained at about 350 fmol/mg DNA from 32 days until 1-2 days pre-partum when it increased to about 650 fmol/mg DNA (P < 0.05). Estradiol binding in endometrium showed an inconsistent pattern and chorion binding for both progestin and estradiol was low and unremarkable. We conclude that there is a potential for decreased progesterone effect in myometrium at about one week before delivery and increased estrogen action in that tissue immediately before delivery.